Roaming communication units utilizing a communication network made up of a central hub controller linked to area-wide system controllers are known. Roaming capabilities typically allow the communication unit to maintain communication with the network while roaming from system to system, but do not encompass talkgroup synchronization as the communication unit moves from one system to another.
U.S. Pat. No. 4,833,701, issued to Comroe et al. and assigned to Motorola Inc., is entitled "Trunked Communication System with Nationwide Roaming Capability" (hereinafter "Comroe"). It discloses a method and system for permitting communication units moving from one communication system area to another to obtain roaming IDs and to continue to operate in the second system area under an assigned roaming ID. It also discloses that a roaming communication unit transmits its individual ID and its home system ID to the local computer for the second system when requesting a roaming ID. It further discloses that the local computer assigns and cross-references a roaming ID to the communication unit ID, transmits the roaming ID to the communication unit, and transmits the communication unit ID and local system ID to the hub controller.
In the Comroe patent, each roaming ID is assigned to the communication unit without cross-referencing to a previously assigned talkgroup. There nevertheless exists a need for a trunked communication system that facilitates continued communication with the same talkgroup as a roaming communication unit leaves one system and enters another.